1. Field of the Invention
The present invention relates to an image supporting material for use in an image forming apparatus such as a copying machine or a printer, and more particularly to an image supporting material adapted for use in color image formation for example by an electrophotographic process, a method for use thereof and an improvement in an image forming apparatus utilizing the same.
2. Background Art
For a color image formation in a prior color image forming apparatus for example by an electrophotographig process, following image forming steps are usually adopted.
A light reflected from an illuminated original is color separated by a color scanner and subjected to image processing and color correction in an image processing apparatus to obtain image signals of plural colors, each of which is converted into a modulated laser beam for example with a semiconductor laser. An image bearing member, utilizing an inorganic photosensitive material such as selenium or amorphous silicon or an organic photosensitive material including a phthalocyanine pigment or a bisazo pigment as a charge generation layer, is irradiated with such laser beams in succession for respective colors to form plural electrostatic latent images. Such plural electrostatic latent images are developed in succession with charged 4-color toners of Y (yellow), M (magenta), C (cyan) and K (black). The developed toner images are transferred from the image bearing member utilizing the inorganic or organic photoconductive material onto an image supporting material and fixed in a fixing apparatus of heat-pressurizing method. A color image is thus obtained on the image supporting material.
The color toner employed in the above-described process is constituted of particles of an average particle size of 1 to 15 μm prepared by dispersing a colorant in a binder resin such as a polyester resin, a Styrene/acrylic copolymer, or a styrene/butadiene copolymer, on which deposited are fine particles of an average particle size of 5 to 100 nm, for example inorganic fine particles such as of silicon oxide, titanium oxide or aluminum oxide or organic fine particles such as of PMMA or PVDF.
The colorant for Y (yellow) color can for example be benzidine yellow, quinoline yellow or Hanza yellow, that for M (magenta) color can be rhodamine B, rose Bengal or pigment red, that for C (cyan) color can be phthalocyanine blue, aniline blue or pigment blue, and that for K (black) color can be carbon black, aniline black or a mixture of color pigments.
As the image supporting material mentioned above, there is being utilized a plain paper principally constituted of pulp, a coated paper coated with a resin mixed with a white pigment or the like on a plain paper, or a white film constituted of a resin such as polyester mixed with a white pigment.
Particularly for forming an image of a high luster equivalent to that of a silver halide-based photograph, it is known preferable, as described in JP-A-2000-010329, JP-A-2000-003060 and JP-A-2000-091212, to use an image supporting material having a layer of a thermoplastic resin of a certain thickness on a plain paper, a coated paper or a white film.
Also for printing a photographic image, an image supporting material of a large thickness is generally preferred.
In the aforementioned transfer step, there is known a method of providing a transfer roller or a transfer belt, prepared for example with a dielectric material, in an opposed relation to the image bearing member constituted for example of a photosensitive member, causing an image supporting material to be adhered in advance on the transfer roller or the transfer belt and applying a bias to the transfer roller or providing a transfer member (such as a transfer corotron, a biased transfer roller or a biased transfer brush) behind the transfer belt, to apply an electric field of a polarity opposite to that of the toner from the transfer roller or from the back of the transfer belt, thereby electrostatically transferring the toner images one by one onto the image supporting material.
In the aforementioned transfer step, there is also known a method of providing an intermediate transfer member for example of a belt shape, prepared for example with a dielectric material, in an opposed relation to the image bearing member constituted for example of a photosensitive member, then applying an electric field of the polarity opposite to that of the toner by a predetermined primary transfer member (such as a transfer corotron, a biased transfer roller or a biased transfer brush) to transfer the toner images formed on the image bearing member one by one onto the intermediate transfer member, thereby once forming a color toner image on the intermediate transfer member, and applying an electric field of the polarity opposite to that of the toner by a predetermined secondary transfer member (such as a transfer corotron, a biased transfer roller or a biased transfer brush) from the back of the base material to electostatically transfer the color toner image onto the base material.
Also in the aforementioned fixing step, there are known a heat-pressure fixing method in which a heat source such as an incandescent lamp is incorporated in a pair of mutually pressed fixing rollers and the image supporting material bearing the transferred color toner image is passed between such paired fixing rollers whereby the color toner is thermally fused and fixed to the image supporting material, and a cool-peeling fixing method in which a fixing belt bearing a superficial releasing layer such as of silicone rubber is supported by plural tension rollers and, a pair of fixing rollers are provided in mutually opposed positions across the fixing belt and a heat source such as an incandescent lamp is incorporated in the paired fixing rollers, wherein the image supporting material bearing the transferred color toner image is superposed with the fixing belt, and passed between the paired fixing roller to fix the toner image by heat and pressure, and the color toner image is separated after cooling thereof from the fixing belt, thus achieving fixation of the color toner image to the base material.
The latter fixing method is known preferable particularly for forming an image of a high gloss equivalent to a silver halide-based photograph. Also a uniform high gloss, irrespective of the image density, can be obtained by combining the latter fixing method and a base material provided with the aforementioned thermoplastic resin layer.
The above is disclosed in JP-A-2000-010329, JP-A-2000-003060, JP-A-2000-091212 and JP-A-2001-117259.
In such image forming apparatus, in case of adopting the image supporting material with the aforementioned thermoplastic resin layer as a base material, a white PET film or a coated paper results in an extremely expensive image supporting material though the image quality can be improved. On the other hand, an inexpensive plain paper is associated with a technical drawback of being unable to obtain a satisfactory image quality.
Also in case the aforementioned thermoplastic resin is principally formed by an amorphous resin such as polyester, polystyrene or polyacrylic resin, there is encountered a technical drawback that a low-temperature fixing property, a heat resistance and a mechanical strength cannot be satisfied at the same time.
More specifically, in order to reduce the energy consumption in the image formation, a low-temperature fixing property is essential, for which a reduction in the molecular weight of the resin and a decrease in the glass transition point thereof are effective measures.
On the other hand, an image of a smooth surface as in a photograph may cause a blocking phenomenon (surfaces being stuck unseparably or damaged even when separated) when let to stand in a state where the front surface of the image is in contact with the front surface or the rear surface of another image, or with a material of an album, in a high temperature environment such as a storage in an automobile or in a warehouse in summer or a transportation in a ship bottom.
In such case, for improving the durability at a high temperature, namely the heat resistance, an elevation of the glass transition point or a high molecular weight is effective.
Also a strength against a image bending, namely mechanical strength, is an important point. An increase in molecular weight is an effective measure for improving mechanical strength.
Thus the measures for improving mechanical strength and the heat resistance are contradictory to those for improving the low-temperature fixing property. Also it is necessary to employ a higher fixing temperature particularly in case of preparing an image of a high gloss as in a silver halide-based photograph, so that it becomes further difficult to meet the three requirements at the same time.
Also the JP-A-2001-117259 proposes to use an image supporting material coated with a crystalline polyester resin. In such case, the low-temperature fixing property and the storage property can be improved, but the mechanical strength is reduced. Also in comparison with a silver halide-based photograph, such image supporting material is inferior in luster and whiteness, and cannot provide a satisfactory photographic quality by forming a color toner image thereon.
Therefore, the present inventors have tried to use an image supporting material having, on a base paper composed at least of a pulp material with a basis weight of 150 to 200 g/m2, a light diffusion layer (corresponding to a light scattering layer) formed by dispersing a white pigment such as titanium oxide by about 30 wt. % in a polyolefinic resin such as polyethylene.
Such image supporting material can be produced relatively inexpensively, and can provide an image with a high whiteness.
In this case, however, there is encountered a technical drawback that a preferred surface structure of a high gloss uniform over the entire image cannot be obtained because the light diffusion layer at the surface of the image supporting layer has a high melt viscosity at the fixing step.